Composite electroless coatings are a new generation of composite which can be derived via electroless plating techniques. The following patents and article reflect the state of the art, the techniques which are used, as well as those particulate matter which may be incorporated within the electroless plating matrix: U.S. Pat. Nos. 3,617,363; 3,674,447; 3,753,667; Reissue 29,285; R. Barras et al, "Electroless Nickel Coatings-Diamond Containing", Electroless Nickel Conference, Cincinnati, Ohio, November, 1979. These patents and publication are included herein by reference.
Though electroless plating may be applied to a wide variety of substrates, the coating of metallic substrates is of great technological interest for achieving any of several properties on the initial substrates (e.g., corrosion protection, wear resistance, frictional properties, etc.). Plating may be carried forth on non-conductor and semiconductor type substrates as well. Though the mechanism of composite electroless plating is not fully understood, it is believed that the insoluble particulate matter suspended within the electroless plating composition is entrapped and bound during the electroless plating process build-up. For an effective entrapment, the insoluble particle must attach itself to the surface and permit the conventional electroless plating process to proceed without interruption of the plating process.
It is therefore recognized, since the particulate matter does not appear to participate in the actual (basic) mechanism (see (1) Lukes, Plating, 51, 969 (1964); (2) N. Feldstein et al, J. Electrochem. Soc., 118, 869 (1971); (3) G. Salvago et al, Plating, 59, 665 (1972)) of the conventional electroless plating but rather is entrapped, that it is therefore essential that there be a high probability for the particulate matter to "stick" to the surface and result in fruitful entrapment rather than contacting the surface and falling off into the bulk solution. It is also recognized that the electroless metal or alloy matrix provides "cement" for the entrapment of the particulate matter. Moreover, it is undesirable for the particles to become autocatalytic; hence, the electroless plating bath used must be stabilized and kept clean.
In reviewing the prior art, the following observations are noted.
(1) In U.S. Pat. Nos. 3,753,667 and 3,562,000 the use of specific particulate matter has been demonstrated along with the suggestion of mixture of particles of a different chemical nature.
(2) Hubbell, in a review article, Plating and Surface Finishing, Dec. 1978, pp. 58-62, has pointed out the general range of which particulate matter can be deposited. Similarly, Safina et al, in Zashchita Metallov, Vol. 14, No. 4, pp. 504-506, July/August 1978, have shown that various particles can be codeposited along with electroless metal deposition; however, in each case only a single nominal size is employed though each size contains a distribution of particle sizes.
(3) In British Patent 1,041,753 composite electroless plating of selected particulate matter is shown (e.g., PVC and Al.sub.2 O.sub.3) along with mixtures of particles of either the same chemical nature or varied chemical nature. However, the U.K. patent stipulates that in the case of admixture of two distinct nominal sizes, the size ratio of the smallest size be no more than one tenth of the larger size particle used. In the U.K. patent the examples for admixture of particles show ratios which are greater than 1:10 (small to large).
(4) Sharp, in an article "Properties and Applications of Composite Diamond Coating", p. 121, (1974) has pointed out the importance of particle size. He has noted that a 3-micron diamond is used more frequently as a compromise for a low surface roughness requirement. This compromise is made at the expense of wear, as is demonstrated in great detail in Reissue Patent 29,285.
(5) The problem of creating a smooth, friendly, composite coating has been noted in Machine Design, Nov. 24, 1977 by D. R. Dreger.
In any event, some of the problems associated with composite electroless plating, particularly as applied to surface roughness, have been recognized but the present findings and solution have not been available heretofore, especially in permitting the use of large particles (e.g., 6 micron or greater) while yet attaining a lower surface roughness with greater ease.
None of the above art provides any appreciation of the present invention in which two nominal sizes of particulate matter are combined, thereby resulting in a smoother surface (as-plated) as well as requiring less energy in attaining a lower level of surface finish (smoothing). Though there are a wide variety of metals and alloys that can be electrolessly plated, commercial applications are primarily focused upon the metals selected from the group of nickel, cobalt and copper. Depending upon the nature of the reducing agent used (e.g., hypophosphite vs. dimethyamine borane), other alloying constituents may be present (e.g., phosphorus vs. boron). For descriptions of the state of the art in electroless plating see "Modern Electroplating", 3rd Edition, John Wiley & Sons, F. A. Lowenheim, Editor, Chapter 31.
In general, in the present invention, particles in the size range of 0.5 to 100 microns may be contemplated, though commercial practices are more limited to 0.5 to 10 microns in size. It has generally been the practice to select the desired nominal particle size with a narrow particle size distribution. In most applications, 15 to 30% by volume has been used, though it is possible, particularly with higher temperature and/or high bath load concentration, to achieve particle loading within the deposit approaching 50%.
In the case of diamond particulate matter, especially diamond of a polycrystalline nature (manufactured by an explosion process), preferred particles may be selected in the range of 1 to 9 microns in size. The actual nominal size depends upon the ultimate application.
The hardness of Ni-P type deposits can reach approximately 69 Rockwell C units with heat treatment, as is well known in the art. In the case of nickel-boron type deposits, hardness values of 1050 VHN.sub.50 can be reached with heat treatment.
The inclusion of particulate matter inherently increases the roughness of the coating. In certain applications (e.g., texturing of yarn utilizing friction texturing discs), a decrease in final surface roughness is most necessary to insure a minimum of damage to the yarn. The final degree of smoothness or surface finish for usage is becoming increasingly important with the ongoing improvements and increased speed of commercial machines.
It is thus the prevailing practice to smooth (e.g., by brushing) the outer surface of the composite coating prior to usage. However, smoothing is a tedious and expensive process, especially for composites containing wear-resistant particles, particularly in a hard matrix. Accordingly, it is thus highly desirable to provide coatings which would preserve their wear-resistance properties while requiring a minimum amount of brushing time (or any other mechanical smoothing technique, e.g., blasting, honing, tumbling, etc.).